Method for producing steam generator tubes

ABSTRACT

A method for producing steam generator tubes is provided. The method is intended to allow a technically particularly simple production process and at the same time allow particularly high flexibility with regards to the materials that may be used to achieve a particularly high efficiency of a steam generator. For this purpose, an insert is fixed in slots of a former shaft, the former shaft with the insert is introduced into a steam generator tube, the fixing of the insert on the former shaft is released and the former shaft is removed again from the steam generator tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2011/057426, filed May 9, 2011 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 10164426.8 EP filed May 31, 2010. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for producing steam generator tubes.

BACKGROUND OF INVENTION

A steam generator is a closed, heated vessel or a pressure pipe or tube system which serves the purpose of producing steam at high pressure and high temperature for heating and operational purposes (e.g. for the operation of a steam turbine). With particularly high steam capacities and pressures, such as, for example, during energy production in power stations, water-tube boilers are used in which the flow medium, usually water, is located in steam generator tubes. Water-tube boilers are also used in applications involving the combustion of solids, since the combustion chamber in which the heat generation takes place through combustion of the respective raw material can be configured at will by the arrangement of tube walls.

A steam generator of said type designed in the manner of a water-tube boiler accordingly comprises a combustion chamber, the encompassing wall of which is formed at least partially by tube walls, i.e. steam generator tubes which are welded so as to be gas-tight. On the flow medium side, these steam generator tubes, as evaporation heating surfaces, initially form an evaporator into which unevaporated medium is introduced and evaporated. The evaporator is in this case usually arranged in the hottest area of the combustion chamber. Arranged downstream of this, on the flow medium side, where appropriate, is a device for separating water and steam, and a superheater in which the steam is heated further above its evaporation temperature in order to attain a high degree of efficiency in a following thermal engine such as, for example, a steam turbine. Connected upstream of the evaporator on the flow medium side there can be a preheater (referred to as an economizer), which preheats the feedwater by making use of waste or residual heat and thereby likewise increases the degree of efficiency of the system as a whole.

For evaporator heating surfaces in steam generators, smooth tubes or tubes with internal ribbing are used. Tubes with internal ribbing are used when the aim is to impose a swirl on the flow of the flow medium in the steam generator tubes, which has the effect of imparting a higher velocity to the fluid on the inner surface of the steam generator tubes. The use of tubes with internal ribbing may in this case be necessary for various reasons, such as, for example, with a low mass flow density of the evaporator under full load. Even at high heat flow densities, the use of tubes with internal ribbing may be necessary (in drum-type boilers, for example): In this situation there is the risk of film boiling, i.e. a film of steam forms on the inside of the steam generator tubes, which, in contrast with well-mixed fluid, has a high heat-insulating effect in the event of bubble or nucleate boiling. The consequence of this is that, while the heat flow density remains the same, the wall temperature may rise sharply, which can lead to the destruction of the heating surfaces. Not least, the use of tubes with internal ribbing can avoid flow layering (separation of the water and steam phase) in normal load operation (such as, for example, at minimum load in helically-tubed steam generators).

According to the prior art, the internal ribbing of the tubes is produced in a cold-drawing process. According to present knowledge, tubes with internal ribbing can only be manufactured with materials with a maximum chromium content of 5%. If the use of tubes with internal ribbing made of alloy steels with a higher chromium content is necessary, due, for example, to a further rise in the steam parameters in order to raise the degree of efficiency, then the internally ribbed tubes cannot be manufactured with the processes available today.

SUMMARY OF INVENTION

The object underlying the invention is therefore to disclose a method for producing steam generator tubes which allows for a technically particularly simple manufacturing process and at the same time permits a particularly high level of flexibility in respect of the materials which can be used in order to achieve a particularly high degree of efficiency of a steam generator.

This object is achieved according to the invention in that an insert is fixed in grooves of a former shaft, the former shaft with the insert is introduced into a steam generator tube, the fixing of the insert on the former shaft is released, and the former shaft is removed again from the steam generator tube.

The invention is based here on the consideration that a particularly high level of flexibility in respect of the materials which can be used could be achieved in that the internally ribbed tubes are not produced in one integrated manufacturing process, but rather that a subsequent introduction of swirl-inducing inserts into smooth tubes should be carried out. This enables the materials of steam generator tubes and inserts to be selected independently of one another. A particularly simple production of the insert can be achieved in this case with the aid of a former shaft. A negative mold of the insert is introduced into the former shaft such that the production of the insert can be carried out by simple integral molding. A former shaft of said type additionally allows for particularly easy fitting of the insert in steam generator tubes in that the insert is fixed on the former shaft, the latter is introduced with the integrally molded insert into the steam generator tube, the fixing is released there, and the former shaft is removed again from the steam generator tube.

In an advantageous embodiment of the method, the insert is fixed to the former shaft with a pretension. As a result of such pretension it is possible for the insert, after release of the fixing to the former shaft, to be lifted out of the grooves of the former shaft and therefore fixed independently in the steam generator tube, without further manual fixing being necessary.

In a further advantageous embodiment, a number of grooves are introduced in the manner of helix into the former shaft. A former shaft of this type naturally produces a helical-shaped insert, which is particularly well-suited for inducing swirl in the flow medium.

In an additional advantageous embodiment, the former shaft is rotated out of the steam generator tube after the release of the fixing of insert and former shaft. This is possible due to the symmetry of the helix, and simplifies the removal of the former shaft from the steam generator tube provided with the insert. This allows for an even simpler manufacturing process of an internally ribbed steam generator tube.

Advantageously, in this arrangement, the grooves of the former shaft are implemented in a conical design. The conical implementation ensures easier joining and backward rotation of the former shaft when it is removed from the steam generator tube.

In a particularly advantageous embodiment, the insert comprises a number of wires. Specifically, as a result of the production of the insert by means of a former shaft which comprises a number of grooves, the insert can be manufactured in a particularly simple manner by appropriate shaping of wires into the grooves. This means not only a particularly economical but also a technically simple production of an insert for a steam generator tube.

In an advantageous embodiment, a steam generator tube manufactured in accordance with the method described hereintofore is used in a steam generator.

The advantages achieved with the invention consist in particular in that through the introduction of an insert into a steam generator tube by means of a former shaft a particularly simple technical solution for the production and positioning of an insert is now provided, which solution allows tubes which are internally ribbed by means of an insert to be manufactured using alloy steels with higher chromium content which are suited to particularly high steam parameters and therefore a particularly high degree of efficiency of a steam generator. The production of the insert can therefore be achieved particularly economically, because the former shaft can be rotated completely out of the tube again and used for the production of the next insert. The significant cost advantages in comparison with cold-drawn tubes with internal ribs make the production method according to the invention attractive even in the case of materials having less than 5% chromium.

By the use of a former element in the form of grooves on a former shaft, the insert has low tolerances in respect of its geometry in the assembled state. As a result of assembly with the former shaft, moreover, no cross-braces or similar ancillary devices are needed in order to ensure the desired geometry of the swirl insert in the assembled state. Swirl inserts can be produced with the aid of a former shaft in commercially available lengths (e.g. 12 m).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail with reference to a drawing, in which:

FIG. 1 shows a former shaft for carrying out the method according to the invention,

FIG. 2 shows a magnified view of the former shaft in section,

FIG. 3 shows a flow diagram of the method according to the invention, and

FIG. 4 is a schematic diagram of a once-through steam generator in a tower design format.

Like parts are labeled with the same reference numerals in all the figures.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a former shaft 1 which is provided in a main area 2 with grooves 4 running around the shaft in helical fashion. In this case the grooves 4 are arranged in the form of a threefold helix around the cylindrical body of the former shaft 1. Depending on the desired profile of the insert which is to be produced, the grooves 4 can also be embodied as a helix having almost any desired number. Depending on the requirement, the desired grooves 4 can be produced individually (number, pitch, dimensions) by means of a metal-cutting machining process. The length of the main area 2 of the former shaft 1 is slightly greater than the smooth tube into which the insert is intended to be introduced. The main area 2 of the former shaft 1 is followed in the axial direction by a connection area 6 which is not provided with grooves 4 and which serves to handle the former shaft 1 during the method according to the invention.

FIG. 2 shows the former shaft 1 in section, in a magnified representation. The profile of the grooves 4 incorporated into the former shaft 1 can be identified at the cut face 8. The grooves 4 have a conical profile. Such a profile allows for a simpler joining and backward rotation of the former shaft 1 during the manufacturing process of the insert (not shown in greater detail) for a steam generator tube.

FIG. 3 shows a schematic diagram depicting the individual steps of the production process according to the invention. In step A, one or more wires are inserted into the former shaft 1, according to the number of grooves 4 which are to be introduced into it. This results in the production of the one-pitch or multi-pitch swirl insert. In step B, the wire or wires is/are fixed to the former shaft 1 under pretension. In step C, the former shaft 1 is introduced into a steam generator tube which is smooth on the inside, and the insert is in this way positioned in the steam generator tube.

In step D, the fixing of the insert formed of wires on the former shaft 1 is released. As a result of the preceding fixing with pretension in step B, the release of the fixing leads to a lifting of the insert out of the grooves 4. In step E, the former shaft 1 is rotated out of the steam generator tube in the manner of a screw, along the helical-shaped grooves 4. This is assisted by the conical shape of the grooves 4. As a result of the release of the fixing and by the reverse rotation of the former shaft 1, the insert is released from the former shaft 1, and, assisted by the internal stress, comes into firm contact with the inner wall of the steam generator tube. The former shaft 1 is rotated completely out of the tube, and can be used in step F for the production of the next insert.

The once-through steam generator 10 according to FIG. 4 is implemented in a tower design format and as a two-pass steam generator. It has an encompassing wall 12 which, at the lower end of the first gas pass formed by it, transitions into a funnel-shaped base 14. The encompassing wall 12 is formed in a lower area, or evaporator area, from evaporation tubes 16, and in an upper area, or superheater area, from superheater tubes 16′. The evaporator tubes 16 and superheater tubes 16′ are joined to one another, for example welded, in a gas-tight manner on their longitudinal sides. The base 14 comprises a discharge aperture 18 for ash (not shown in greater detail).

The evaporator tubes 16 of the encompassing wall 12, through which a flow medium, in particular water or a water-steam mixture, flows from bottom to top, are connected at their inlet ends to an inlet manifold 20. On the outlet side the evaporator tubes 16 are connected by means of a water separation system (not shown in greater detail) to the superheater tubes 16′ which follow downstream on the flow medium side.

The evaporator tubes 16 of the encompassing wall 12 form an evaporation heating surface 22 in the section of the gas pass located between the inlet manifold 20 and the water separation system. This is followed by a reheater or superheater heating surface 24 formed from the superheater tubes 16′. In addition, arranged in the second gas pass 26, through which the hot gases flow downward, and in the transverse pass 28, connecting it on the hot gas side to the first gas pass, are further heating surfaces 30, only represented schematically, such as an economizer and convective superheater heating surfaces.

Mounted in the lower area of the encompassing wall 12 are a number of burners for a fossil fuel, in each case in an aperture 32 of the encompassing wall 12. Four apertures 32 are visible in FIG. 1. At an aperture 32 of this kind the evaporator tubes 16 of the encompassing wall 12 are curved so as to circumvent the respective aperture 32, and run on the outside of the vertical gas pass. These apertures can, for example, also be provided for air nozzles.

As a result of the use of steam generator tubes with internal ribbing in the steam generator 10, produced according to the described method, it is possible to make use also of steels having a chromium content of more than 5% for their manufacture. Such steam generator tubes are suitable for particularly high steam parameters, and so enable a particularly high degree of efficiency of a steam generator to be achieved. 

1-8. (canceled)
 9. A method for producing steam generator tubes, comprising: fixing an insert in a plurality of grooves of a former shaft; introducing the former shaft with the insert into a steam generator tube; releasing the fixing of the insert to the former shaft; and removing the former shaft again from the steam generator tube.
 10. The method as claimed in claim 9, wherein the insert is fixed to the former shaft with a pretension.
 11. The method as claimed in claim 9, wherein the plurality of grooves are introduced in a helical shape into the former shaft.
 12. The method as claimed in claim 11, wherein the former shaft is rotated out of the steam generator tube.
 13. The method as claimed in claim 9, wherein the plurality of grooves are implemented as conical.
 14. The method as claimed in claim 9, wherein the insert comprises a plurality of wires.
 15. A steam generator tube, comprising: a steam generator tube produced by means of the method as claimed in claim
 9. 16. A steam generator, comprising: a steam generator tube as claimed in claim
 15. 